The Heron Moon [Mystery Booster 2 Playtest Cards]

{T}: Add {C}.
{1}, {T}: Exile the bottom card of target opponent's library.
Whenever one or more cards an opponent owns are put into exile, put a release counter on The Heron Moon. Then if it has thirteen or more release counters on it, sacrifice it and create a copy of Emrakul, the Promised End. Cast it without paying its mana cost.